144 METALLURGY OE IRON" AND STEEL.
shown in Fig. VIII-G-. It is not uncommon for large works to have one or more furnaces so arranged that the entire top of the furnace is removable, thus giving an opportunity to dispose of heavy sculls and pieces that cannot easily be broken, but the furnace cools so much during this process of taking off the roof that considerably more fuel is,used than in the ordinary types, and the roof does not last as long, owing to the severe strains in cooling and heating.
SEO. Vlllf.—Ports.—The working of the furnace depends very much upon the arrangement of the ports through which the gases come and go. The gas should enter below the air, because, being lighter, mixture is facilitated, and because this arrangement does not expose the metal on the hearth to a stratum of hot air and cause excessive oxidation. The point where the two gases meet should be about five feet from the metal; if much less than this, combustion can hardly begin before it is checked by contact with the cold stock; if much more, and if the burning mixture is conducted between confining walls, the brickwork will be melted. Both gas and air should enter the combustion chamber under a positive pressure, forcing them into contact with each other and throwing the resultant flame across the furnace in such a way that the draught of the stack on the outgoing end can 'pull it down through the ports without its impinging upon the roof. A prevalent idea among furnacemen is that the draught of the stack pulls the gases into the furnace; but this is entirely wrong. They are not pulled; they are pushed in by the upward force of the white-hot vertical port on the incoming end, and where this force is not sufficient, as in horizontal chambers, a blower should be used as an auxiliary.
The figures in Sec. VIIIc will show the different ways in which the port question has been answered. In Fig. VIII-C the portion of the construction next to the furnace is a removable cage containing the arch that divides the gas and air. When this arch is worn back this section can be removed by a crane and replaced by a new one, the whole operation not taking over one hour, and not interrupting the operation of the furnace. This system is the device of C. B. Stafford. The drawing of the furnace at Duquesne shows how simple the problem becomes when natural gas is used.
SEO. Vlllg.—Valves.—The amount of gas and air admitted to the chambers is regulated by some form of throttle valve. Eevers-